As the size of mobile wireless communications devices (e.g., cellular devices) decreases, so too does the allowable space for the device antenna. In the near future, a mobile handset, which may operate over multiple frequency bands to provide various communication services (e.g., GSM 850/900/1800/1900 and UNITS 2100) may be required to accommodate more than one antenna to achieve such wideband operation, as well as to provide desired beam forming and/or enhance communications system capacity. As a result, designing small antennas that can meet these technical challenges can be difficult. See, e.g., Wen, “Physical Limitations of Antennas,” IEEE Transactions on Antennas and Propagation, vol. 51, no. 8, pgs. 2116-2123, 2003.
Internal planar inverted-F antennas (PIFAs) are commonly used in wireless handset devices. However, one drawback of typical PIFA antennas is that they have a relatively limited (i.e., narrow) frequency bandwidth. See, e.g., Liu et al., “Dual-Frequency Planar Inverted-F Antenna,” IEEE Transactions on Antennas and Propagation, vol. 45, no. 9, pgs. 1451-1457, October 1997; Rowell et al., “A Compact PIFA Suitable for Dual-Frequency 900/1800-MHz Operation,” IEEE Transactions on Antennas and Propagation, vol. 46, pgs. 586-598, April 1998; and Chiu et al, “Compact Dual-Band PIFA with Multi-Resonators, Electronic Letters,” vol. 38, pgs. 538-554, 2002. To enhance the bandwidth of a PIFA, these antennas are sometimes combined with other broadband technologies, such as parasitic elements or multi-layered structures. See, e.g., Quo et al., “Miniature Built-In Quad-Band Antennas for Mobile Handsets, IEEE Antennas and Wireless Propagation Letters,” vol. 2, pgs. 30-32, 2003. However, these approaches may result in a larger PCB surface area to implement, as well as complicating the manufacture process. Moreover, it may be somewhat challenging to tune the resonant frequencies of a PIFA with resonant branches.
Another form of antenna, the monopole antenna, typically has a relatively wide bandwidth as compared with that of a PIFA. However, a significant drawback of typical monopole antennas is that they require a relatively large surface area (i.e., they are larger) than a comparable PIFA. Another drawback of monopole antennas is that, due in part to the size constraints, when they are implemented in a handheld device they are typically implemented as external antennas, which results in an undesirable form factor for users. PIFAs, on the other hand, are relatively easy to implement as internal antennas.
Even so, another advantage that a monopole antenna has over the PIFA, in addition to its wideband response, is its isolation from the surrounding environment, and, more specifically, the ground plane. Monopole antennas are also comparatively simpler, and have a relatively low profile.
One exemplary monopole antenna arrangement is set forth in U.S. Pat. No. 6,054,955 to Schlegel, Jr., et al. This patent is directed to an antenna arrangement for use in the housing of a portable communications device. The antenna arrangement includes a pair of spaced folded monopole antennas. Each antenna includes a first printed circuit board having a conducting surface that forms a ground plane. Mounted on the first circuit board is a second printed circuit board having a right-angled strip of conducting material, which forms a folded monopole radiating element. The folding of the monopole reduces its height, to thereby enable it to fit into small casings and the like. To compensate for the effects of the folded monopole on the electrical match, frequency bandwidth and electromagnetic fields, a shunt inductance is introduced between the monopole and the ground plane. The antennas are mounted within cavities that can be lined or coated with metallic material to improve the radiation patterns of the antennas and isolate them from the electronic components of the communications system.
Despite the existence of such antenna arrangements, further advancements in monopole antenna structures for mobile wireless communications device may be desirable in some applications.